This invention relates to photocathodes and more particularly to a photocathode comprising potassium, cesium, rubidium and antimony, and to methods of forming such a photocathode.
Some prior art photocathodes that comprise an initial deposit of antimony and one or more alkali metals include the cesium-antimony (Cs.sub.3 Sb) photocathode, the potassium-cesium-antimony (K.sub.2 CsSb) bialkali photocathode, the sodium-potassium-antimony (Na.sub.2 KSb) bialkali photocathode and the potassium-sodium-cesium-antimony ((Cs)Na.sub.2 KSb) multialkali photocathode. The cesium-antimony, or S-11 photocathode, has been supplanted in most applications by one of the above-mentioned bialkali photocathodes, each of which has a higher spectral sensitivity and quantum efficiency than the S-11 photocathode. With the exception of the potassium-sodium-cesium-antimony photocathode, commonly referred to as an S-20 photocathode, the photocathodes of the above-mentioned type are characterized by relatively high resistivity.
When a photocathode is deposited directly upon an insulating substrate, such as a glass faceplate of a photomultiplier tube, it is usually caused to overlap, at its periphery, a conductive layer, e.g., an evaporated aluminum film, connected to a source of suitable electrical potential, such as ground. Such a conductive layer serves to replenish emission electrons lost by the photocathode in operation. However, because of the relatively high resistivity of each of the above-mentioned bialkali photocathodes a relatively large voltage gradient is produced across the photocathode during operation. This voltage gradient distorts an electrostatic focusing field adjacent to the photocathode so as to adversely affect the focusing function of such field with consequent distortion in the output signal of the photomultiplier tube.
In certain applications where a photocathode is subjected to an intense source of radiation, such as light emitted by a scintillator, a photocathode having a high resistivity is not suitable because of the appreciable voltage gradient across the photocathode with its attendant adverse effect on tube performance.
In such an application either a transmissive conductive substrate must be dispersed on the glass faceplate between the photocathode and the radiation source to lower the resistivity of the photocathode, or a photocathode more conductive than a bialkali photocathode must be employed.
While the conductivity of the S-20 multialkali photocathode is satisfactory for use with an intense source of radiation, the S-20 -photocathode fabrication process is slow and costly. A typical cathode processing schedule for an S-20 multialkali photocathode requires a complex sequence of processing steps including alternately evaporating the alkali materials potassium, sodium and cesium with antimony until a maximum level of photocathode sensitivity is achieved. Such a schedule is described in U.S. Pat. No. 3,658,400 issued to F. A. Helvy on Apr. 25, 1972, and entitled "Method of Making a Multialkali Photocathode with Improved Sensitivity to Infrared Light and a Photocathode Made Thereby."
Photocathodes comprising rubidium as one of the constituents are well known in the art. For example, the superficially oxidized rubidium-cesium-antimony bialkali photocathode is disclosed by C. W. Morrison, in an article entitled, "Techniques for Producing High Sensitivity Rubidium-Cesium-Antimony Photocathodes", Journal of Applied Physics, Vol. 37, No. 2, Feb. 1966, pages 713-715, and also by A. H. Sommer, in Photoemissive Materials, John Wiley and Sons, Inc., New York, 1968, pages 126-127.
A rubidium-potassium-sodium-cesium-antimony multialkali photocathode is disclosed in U.S. Pat. No. 3,498,834, Rome et al., issued Mar. 3, 1970, entitled "Photoelectric Surfaces and Methods For their Production."
Both the superficially oxidized rubidium bialkali photocathode disclosed by Morrison and Sommer and the rubidium multialkali photocathode disclosed by Rome et al. require complex manufacturing processes and thus the photocathodes produced thereby are subject to considerable variations in quality and high manufacturing costs. Therefore these photocathodes are not suitable for photomultiplier tube applications where a low cost, high performance photocathode is required.
In addition to the above-mentioned prior art rubidium photocathodes, a rubidium-cesium-antimony bialkali photocathode is disclosed in a copending application of A. F. McDonie, Ser. No. 937,567, filed Aug. 28, 1978, entitled, "Rubidium-Cesium-Antimony Photocathode," assigned to the same assignee as the instant invention and now abandoned. The rubidium-cesium-antimony bialkali photocathode disclosed by McDonie provides the advantages of ease of photocathode processing and satisfactory conductivity; however, the spectral sensitivity of the aforementioned rubidium-cesium-antimony bialkali photocathode, in some photomultiplier tubes, is not as high as would be desired.